In wireless communication technologies, such as cellular systems, equalizers are used as a technique for reducing multipath interferences. Equalizers are a technique for reducing interferences by calculating an appropriate synthesis (weighting) coefficient from estimation results of a propagation path (channel), and weighting-adding reception signals using the synthesis coefficient, thereby equalizing reception signals.
Here, for example, a signal y(t) after an equalizer at time t can be expressed using a reception signal x(t) and a synthesis coefficient (tap coefficient) w(i) according to the following Eq. (1.1):
                              y          ⁡                      (            t            )                          =                              ∑            i                    ⁢                                    w              ⁡                              (                i                )                                      ·                          x              ⁡                              (                                  t                  -                  i                                )                                                                        (        1.1        )            
How the synthesis coefficient w(t) is calculated determines characteristics of the equalizer and the processing amount.
Conventional equalizers either calculate the synthesis coefficient w based on the correlation matrix R0 only of the serving cell as expressed by the following Eq. (1.2) (see Non-Patent References 1 and 2 that will be listed later, for example), or calculate the tap coefficient by determining a synthesis correlation matrix by a sum of the correlation matrix R0 of the serving cell and correlation matrixes Ri of non-serving cells (base station number #i) as expressed by the following Eq. (1.3) (see Non-Patent Reference 3 that will be listed later, for example).
                    w        =                                            (                                                R                  0                                +                                                      σ                    2                                    ⁢                  I                                            )                                      -              1                                ·          h                                    (        1.2        )                                w        =                                            (                                                R                  0                                +                                                      ∑                    i                                    ⁢                                      R                    i                                                  +                                                      σ                    2                                    ⁢                  I                                            )                                      -              1                                ·          h                                    (        1.3        )            
In the above Eqs. (1.2) and (1.3), I represents the unit matrix, and σ2 represents the noise. In addition, a correlation matrix R can be calculated by the following Eq. (1.4) using the channel estimation value α(r, k) of the reception antenna number #r and the path number #k:
                              R                                    (                                                r                  ⁢                                                                          ⁢                  1                                ,                                  k                  ⁢                                                                          ⁢                  1                                            )                        ,                          (                                                r                  ⁢                                                                          ⁢                  2                                ,                                  k                  ⁢                                                                          ⁢                  2                                            )                                      =                              ∑            i                    ⁢                                                    α                ⁡                                  (                                                            r                      ⁢                                                                                          ⁢                      1                                        ,                                          i                      +                                              k                        ⁢                                                                                                  ⁢                        1                                                                              )                                            *                        ·                          α              ⁡                              (                                                      r                    ⁢                                                                                  ⁢                    2                                    ,                                      i                    +                                          k                      ⁢                                                                                          ⁢                      2                                                                      )                                                                        (        1.4        )            
Note that the term “correlation matrix R0 of the serving cell” refers to a matrix having, as an element, a correlation value based on a channel estimation value of an incoming signal (path) from a base station (serving cell) with which a wireless communication apparatus having an equalizer is communicating. The term “correlation matrix of anon-serving cell Ri” refers to a matrix having, as an element, a correlation value based on a channel estimation value of an incoming signal (path) from a base station (interference station) other than that serving cell (the definitions of the terms will be used throughout the specification).
Note that well-known techniques regarding equalizers include the techniques described in the Patent References 1 and 2 listed below.
The technique in Patent Reference 1 is directed to provide a CDMA reception apparatus that experiences reduced deterioration of the receiver performance even when the number of concurrent users is increased in the DS/CDMA scheme during fast fading or in a multipath environment, and thus multiple interference elimination equalizers are provided for this purpose. Those interference elimination equalizer are adapted to regard multipath signal components at timings other than specified timing as signals from non-serving cells and eliminates such components, and to syntheses a multipath signal component only at the designated timing.
The technique in Patent Reference 2 is directed to provide a noise power estimation apparatus that can estimate a chip noise power used for a weight calculation for an MMSE equalizer or the like. For this purpose, the apparatus is adapted to determine the noise power by eliminating multipath interference components from the reception power of the pilot signal for each path using the power ratio of pilot signal to data signal for each path, determining a modified reception power of the pilot signal for each path, estimating the total power of the pilot signal and the data signal included in the reception signal based on the modified reception power for the multiple paths and the predetermined power ratio, and subtracting the estimated total power from the total power of the reception signal.                Patent Reference 1: Japanese Laid-Open Patent 1 Publication No. H07-30519        Patent Reference 2: Japanese Laid-Open Patent Publication No. 2005-328311        Non-Patent Reference 1: A. Klein, “Data Detection Algorithms Specially Designed for the Downlink of Mobile Radio Systems”, Proc. of IEEE VTC'97, PP. 203-207, Phoenix, May 1997        Non-Patent Reference 2: 3GPP R4-040680, “HSDPA improvements for UE categories 7 and 8”, Nokia, TSG RAN WG4#33, November 2004        Non-Patent Reference 3: 3GPP R4-060514, “Reference structure for interference mitigation simulations with HSDPA and receiver diversity”, Nokia, TSG RAN WG#39, May 2006        
However, in the case of determining the synthesis coefficient w based solely on the correlation matrix R0 of the serving cell (hereinafter, referred to as a Conventional Example 1), although the processing amount can be reduced, the receiver performance may be deteriorated since the interference components from non-serving cells cannot be reduced. On the other hand, in the case of determining the synthesis coefficient based on the correlation matrix R0 of the serving cell and correlation matrixes Ri of non-serving cells (hereinafter, referred to as a Conventional Example 2), the processing (calculation) amount is increased although the receiver performance is improved. That is, the calculation amount of twofold or higher is required compared to the case based solely on the correlation matrix R0 of the serving cell since a correlation matrixes R should be calculated for the serving cell and each of the non-serving cells.
In addition, since the technique in Patent Reference 1 inadvertently may eliminate multipath signal components at timings other than the specified timing since such components are mistaken as signals from non-serving cell and multipath signal components of the serving cell are accidentally eliminated, or conversely, multipath signal components of non-serving cells may be erroneously included into multipath signal components of the serving cell, which may deteriorate the receiver performance.
In addition, since the technique in Patent Reference 2 is a technique in which the noise power from the serving cell is measured highly accurately without taking multipath signal components from non-serving cells into consideration, the receiver performance may be degraded due to interferences from non-serving cells.